A Spanish organization called Fundacion ANAR, or Aid to Children and Adolescents at Risk, created a bus-stop advertisement in April that features the group’s hotline number for children to report abuse. But by using a process called lenticular photography, the company made the hotline number, and much of the ad’s content, visible only to those under a certain height — presumably children.

Lenticular photography allows companies to create an image in a way that lets viewers see one of several different photos, depending on where they’re standing. In the case of ANAR’s ad, anyone taller than 4 feet 5 inches — the average height of a 10-year-old, according to the group — would see a picture of a boy with an unmarked face and the following message: “Sometimes, child abuse is only visible to the child suffering it.” Anyone under that height would see an image of the boy with a bruised face, the organization’s hotline number (116-111) in white text, and the message, “If somebody hurts you, phone us and we’ll help you” (Huffington Post, 2013).